/*
 * tail command
 *
 *  tail where [file]
 *  where is +/-n[type]
 *  - means n lines before end
 *  + means nth line from beginning
 *  type 'b' means tail n blocks, not lines
 *  type 'c' means tail n characters
 *  Type 'r' means in lines in reverse order from end
 *   (for -r, default is entire buffer )
 *  option 'f' means loop endlessly trying to read more
 *      characters after the end of file, on the  assumption
 *      that the file is growing
 *
 * Copyright (c) 1980 Regents of the University of California.
 * All rights reserved.  The Berkeley software License Agreement
 * specifies the terms and conditions for redistribution.
 */
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <errno.h>

#ifdef pdp11
#define LBIN 16385
#else
#define LBIN 32769
#endif

#undef  BUFSIZ
#define BUFSIZ  8192

struct  stat    statb;
int follow;
int piped;
char bin[LBIN];
int errno;

main(argc,argv)
char **argv;
{
    long n,di;
    register i,j,k;
    char    *arg;
    int partial,bylines,bkwds,fromend,lastnl;
    char *p;

    arg = argv[1];
    if(argc<=1 || *arg!='-'&&*arg!='+') {
        arg = "-10l";
        argc++;
        argv--;
    }
    fromend = *arg=='-';
    arg++;
    if (isdigit(*arg)) {
        n = 0;
        while(isdigit(*arg))
            n = n*10 + *arg++ - '0';
    } else
        n = -1;
    if(!fromend&&n>0)
        n--;
    if(argc>2) {
        (void)close(0);
        if(open(argv[2],0)!=0) {
            perror(argv[2]);
            exit(1);
        }
    }
    (void)lseek(0,(off_t)0,L_INCR);
    piped = errno==ESPIPE;
    bylines = -1; bkwds = 0;
    while(*arg)
    switch(*arg++) {

    case 'b':
        if (n == -1) n = 1;
        n <<= 9;
        if(bylines!=-1) goto errcom;
        bylines=0;
        break;
    case 'c':
        if(bylines!=-1) goto errcom;
        bylines=0;
        break;
    case 'f':
        follow = 1;
        break;
    case 'r':
        if(n==-1) n = LBIN;
        bkwds = 1; fromend = 1; bylines = 1;
        break;
    case 'l':
        if(bylines!=-1) goto errcom;
        bylines = 1;
        break;
    default:
        goto errcom;
    }
    if (n==-1) n = 10;
    if(bylines==-1) bylines = 1;
    if(bkwds) follow=0;
    if(fromend)
        goto keep;

            /*seek from beginning */

    if(bylines) {
        j = 0;
        while(n-->0) {
            do {
                if(j--<=0) {
                    p = bin;
                    j = read(0,p,BUFSIZ);
                    if(j--<=0)
                        fexit();
                }
            } while(*p++ != '\n');
        }
        (void)write(1,p,j);
    } else  if(n>0) {
        if(!piped)
            (void)fstat(0,&statb);
        if(piped||(statb.st_mode&S_IFMT)==S_IFCHR)
            while(n>0) {
                i = n>BUFSIZ?BUFSIZ:n;
                i = read(0,bin,i);
                if(i<=0)
                    fexit();
                n -= i;
            }
        else
            (void)lseek(0,(off_t)n,L_SET);
    }
copy:
    while((i=read(0,bin,BUFSIZ))>0)
        (void)write(1,bin,i);
    fexit();

            /*seek from end*/

keep:
    if(n <= 0)
        fexit();
    if(!piped) {
        (void)fstat(0,&statb);
        /* If by lines, back up 1 buffer: else back up as needed */
        di = bylines?LBIN-1:n;
        if(statb.st_size > di)
            (void)lseek(0,(off_t)-di,L_XTND);
        if(!bylines)
            goto copy;
    }
    partial = 1;
    for(;;) {
        i = 0;
        do {
            j = read(0,&bin[i],LBIN-i);
            if(j<=0)
                goto brka;
            i += j;
        } while(i<LBIN);
        partial = 0;
    }
brka:
    if(!bylines) {
        k =
            n<=i ? i-n:
            partial ? 0:
            n>=LBIN ? i+1:
            i-n+LBIN;
        k--;
    } else {
        if(bkwds && bin[i==0?LBIN-1:i-1]!='\n'){    /* force trailing newline */
            bin[i]='\n';
            if(++i>=LBIN) {i = 0; partial = 0;}
        }
        k = i;
        j = 0;
        do {
            lastnl = k;
            do {
                if(--k<0) {
                    if(partial) {
                        if(bkwds)
                            (void)write(1,bin,lastnl+1);
                        goto brkb;
                    }
                    k = LBIN -1;
                }
            } while(bin[k]!='\n'&&k!=i);
            if(bkwds && j>0){
                if(k<lastnl) (void)write(1,&bin[k+1],lastnl-k);
                else {
                    (void)write(1,&bin[k+1],LBIN-k-1);
                    (void)write(1,bin,lastnl+1);
                }
            }
        } while(j++<n&&k!=i);
brkb:
        if(bkwds) exit(0);
        if(k==i) do {
            if(++k>=LBIN)
                k = 0;
        } while(bin[k]!='\n'&&k!=i);
    }
    if(k<i)
        (void)write(1,&bin[k+1],i-k-1);
    else {
        (void)write(1,&bin[k+1],LBIN-k-1);
        (void)write(1,bin,i);
    }
    fexit();
errcom:
    fprintf(stderr, "usage: tail [+_[n][lbc][rf]] [file]\n");
    exit(2);
}

fexit()
{   register int n;
    if (!follow || piped) exit(0);
    for (;;)
    {   sleep(1);
        while ((n = read (0, bin, BUFSIZ)) > 0)
            (void)write (1, bin, n);
    }
}
